1. Field of the Invention
The present invention relates to a transmitting/receiving unit for use in, for example, a communication system between movable members, and more particularly to a transmitting/receiving unit for use in a communication system between movable members, the transmitting/receiving unit having a signal transmitting portion and a signal receiving portion, the gain of each of which can be controlled, so as to generate an output signal of an accurate level corresponding to only the level of a received input signal.
2. Description of the Prior Art
Hitherto, a transmitting/receiving unit for use in a communication system between movable members has employed a means for controlling the level of the power for transmitting an output signal to correspond to the level of a received input signal. Thus, if the level of the received input signal is too low, the power for transmitting the output signal is increased. If the level of the received input signal is too high, the level of the power for transmitting the output signal is reduced.
FIG. 8 is a block diagram which illustrates an example of the structure of a transmitting/receiving unit of the foregoing conventional type.
Referring to FIG. 8, reference numeral 50 represents a signal transmitting portion, 51 represents a signal receiving portion, 52 represents a wave divider, 53 represents an antenna, 54 represents an adder, 55 represents a first frequency-converter, 56 represents a first automatic gain control (AGC) amplifier, 57 represents a second frequency-converter, 58 represents a RF power amplifier, 59 represents a RF low-noise amplifier, 60 represents a third frequency-converter, 61 represents a second AGC amplifier, 62 represents a fourth frequency-converter, 63 represents an intermediate-frequency amplifier, 64 represents an AGC voltage generator, 65 represents a base-band signal input terminal, 66 represents a base-band signal output terminal, 67 represents a control voltage supply terminal, and 68 represents a first control-voltage supply terminal.
The signal transmitting portion 50 comprises the first frequency-converter 55 for converting a base-band signal to be supplied to the base-band signal input terminal 65 into a first RF signal, the first AGC amplifier 56 for variable-gain-amplifying the first RF signal, the second frequency-converter 57 for converting the amplified first RF signal into an output frequency signal and the RF power amplifier 58 for amplifying the power for transmitting the output frequency signal to a transmission level. The signal receiving portion 51 comprises the RF low-noise amplifier 59 for amplifying a received frequency signal, the third frequency-converter 60 for converting the thus-amplified received frequency signal into a second RF signal, the second AGC amplifier 61 for variable-gain-amplifying the second RF signal, the fourth frequency-converter 62 for converting the thus-amplified second RF signal into an intermediate frequency signal, the intermediate-frequency amplifier 63 for amplifying the intermediate frequency signal to supply the amplified signal to the base-band signal output terminal 66 and for generating DC voltage V.sub.DET corresponding to the output signal level of the intermediate frequency signal, and the AGC voltage generator 64 for amplifying the difference between the DC voltage V.sub.DET and second control voltage V.sub.RREF supplied to the second control voltage supply terminal 67 so as to generate second AGC voltage V.sub.RAGC. The wave divider 52 is connected to an output terminal of the RF power amplifier 58, an input terminal of the RF low-noise amplifier 59 and the antenna 53. The adder 54 sums up the second AGC voltage V.sub.RAGC and first control voltage V.sub.TCONT to be supplied to the first control voltage supply terminal 68 so as to generate first AGC voltage V.sub.TAGC. The first control voltage V.sub.TCONT and the second control voltage V.sub.RREF are used to set the transmission level and the receiving level, respectively.
FIG. 9 is a circuit diagram which illustrates an example of the structure of the first and second AGC amplifiers for use in the conventional transmitting/receiving unit. FIG. 9 illustrates one of a plurality of amplifying stages which respectively form the first and second AGC amplifiers.
Referring to FIG. 9, reference numeral 69 represents a first double-gate MOSFET forming one of amplifying stages of the first AGC amplifier 56, 70 represents a second double-gate MOSFET forming one of amplifying stages of the second AGC amplifier 61. The same elements as those shown in FIG. 8 are given the same reference numerals.
Each of the first and second double-gate MOSFETs 69 and 70 comprises a source S which is grounded, a first gate G1 to which a RF signal to be amplified is supplied, and a drain D which transmits the amplified RF signal. The first double-gate MOSFET 69 has a second gate G2 to which the first AGC voltage V.sub.TAGC is supplied. The second double-gate MOSFET 70 has a second gate G2 to which the second AGC voltage V.sub.RAGC is supplied.
FIG. 10 is a circuit diagram which illustrates another example of the structure of the first and second AGC amplifiers for use in the foregoing conventional transmitting/receiving unit. Also FIG. 10 illustrates one of a plurality of amplifying stages which respectively form the first and second AGC amplifiers.
Referring to FIG. 10, reference numeral 71 represents a first bipolar transistor, 72 represents a second bipolar transistor, 73 represents a third bipolar transistor, 74 represents a fourth bipolar transistor, 75 represents a fifth bipolar transistor and 76 represents a sixth bipolar transistor. The same elements as those shown in FIG. 1 are given the same reference numerals.
The first and second bipolar transistors 71 and 72 form a first difference-amplifying stage to which their emitters are connected commonly. The fourth and fifth bipolar transistors 74 and 75 form a second difference-amplifying stage to which their emitters are connected commonly. The third bipolar transistor 73 is a transistor for a first electric current source and connected between the commonly-connected emitter of the second difference-amplifying stage and the ground. Also the sixth bipolar transistor 76 is a transistor for a second electric current source and connected between the commonly-connected emitter of the second difference-amplifying stage and the ground. The first AGC voltage V.sub.TAGC is supplied to the base of the transistor 73 for the first electric current source, while the second AGC voltage V.sub.RAGC is supplied to the base of the transistor 76 for the second electric current source. Moreover, balanced RF signals are supplied to the bases of a pair of the transistors of the first and second difference-amplifying stages, thus causing amplified RF signals to be transmitted in a balanced manner from collectors.
The thus-constituted conventional transmitting/receiving unit is operated schematically as follows.
The operation of the signal transmitting portion 50 will now be described. The base-band signal supplied to the base-band signal input terminal 65 is converted into the first RF signal in the first frequency-converter 55. The thus-converted first RF signal is variable-gain-amplified in the first AGC amplifier 56. Then, the thus-amplified first RF signal is converted into an output frequency signal in the second frequency-converter 57. The power of the converted output frequency signal is amplified to an output power level in the RF power amplifier 58. Then, the output frequency signal, the power of which has been amplified as described above, is supplied to the antenna 53 through the wave divider 52 and transmitted to the air.
The operation of the signal receiving portion 51 will now be described. The frequency signal received by the antenna 53 is supplied to the RF low-noise amplifier 59 through the wave divider 52 so as to be amplified to a predetermined level. Then, the amplified received frequency signal is converted into the second RF signal in the third frequency-converter 60, the second RF signal being then variable-gain-amplified in the second AGC amplifier 61. Then, the amplified second RF signal is converted into the intermediate frequency signal in the fourth frequency-converter 62. The converted intermediate frequency signal is amplified by the intermediate-frequency amplifier 63 so as to be supplied, as the base-band signal, to the base-band signal output terminal 66. The frequency signal is as well as detected by the intermediate-frequency amplifier 63 so as to be converted into the DC voltage V.sub.DET corresponding to the output level of the intermediate frequency signal. At this time, the DC voltage V.sub.DET is supplied to the AGC voltage generator 64. In the AGC voltage generator 64, the difference between the DC voltage V.sub.DET and the second control voltage V.sub.RREF to be supplied to the second control voltage supply terminal 67 is amplified so that the second AGC voltage V.sub.RAGC is generated. If the level of the received frequency signal, that is, if the output signal level of the intermediate-frequency amplifier 63 is high, the level of the second AGC voltage V.sub.RAGC is lowered to correspond to the foregoing level. If the output signal level of the intermediate-frequency amplifier 63 is low, the level of the second AGC voltage V.sub.RAGC is raised to correspond to the foregoing level.
The second AGC voltage V.sub.RAGC is supplied to the second gate G2 of the second double-gate MOSFET 70 of the second AGC amplifier 61 having the structure shown in FIG. 9 or FIG. 10 or the base of the transistor 76 for the second electric current source. In either structure, the gain of the second double-gate MOSFET 70 or that of the second difference-amplifying stage to which the transistor 76 for the second electric current source is connected is increased if the level of the second AGC voltage V.sub.RAGC is high. The gain is reduced if the level is low. Therefore, if the level of the received frequency signal is high and the output signal level of the intermediate-frequency amplifier 63 is high, the second AGC voltage V.sub.RAGC is lowered, thus causing the gain of the second double-gate MOSFET 70 or that of the second difference-amplifying stage, that is, the gain of the second AGC amplifier 61 to be reduced. If the output signal level of the intermediate-frequency amplifier 63 is low, the second AGC voltage V.sub.RAGC is raised, thus causing the gain of the second double-gate MOSFET 70 or that of the second difference-amplifying stage, that is, the gain of the second AGC amplifier 61 to be increased. It should be noted that a closed loop circuit consisting of the second AGC amplifier 61, fourth frequency-converter 62, intermediate-frequency amplifier 63 and the AGC voltage generator 64 forms an automatic gain control (AGC) loop. Thus, change in the signal level of the base-band signal can be somewhat restricted even if the level of the received frequency signal is changed relatively considerably.
The second AGC voltage V.sub.RAGC is, in the adder 54, added to the first control voltage V.sub.TCONT supplied to the first control voltage supply terminal 68, thus causing the first AGC voltage V.sub.TAGC to be generated at the output of the adder 54. The level of the first AGC voltage V.sub.TAGC is raised/lowered to correspond to the change in the level of the second AGC voltage V.sub.RAGC. When the first AGC voltage V.sub.TAGC is supplied to the second gate G2 of the first double-gate MOSFET 69 forming the first AGC amplifier 56 or to the base of the transistor 73 for the first electric current source, the gain of the first double-gate MOSFET 69 or that of first difference-amplifying stage to which the transistor 73 for the first electric current source is connected, that is, the gain of the first AGC amplifier 56 is, similarly to the gain of the second AGC amplifier 61, increased if the level of the first AGC voltage V.sub.TAGC is high. If the level is low, the gain is reduced. Therefore, if the level of the second AGC voltage V.sub. RAGC is lowered in a case where the level of the received frequency signal is high, the level of the first AGC voltage V.sub.TAGC is lowered. Therefore, the gain of the first AGC amplifier 56 is reduced. If the level of the second AGC voltage V.sub.RAGC is raised in a case where the level of the received frequency signal is low, also the level of the first AGC voltage V.sub.TAGC is raised. As a result, the gain of the first AGC amplifier 56 is increased.
As described above, the conventional transmitting/receiving unit is able to lower or raise the level of the output level of the RF output signal to be transmitted from the signal transmitting portion 50 to correspond to the rise or fall of the level of the frequency signal to be received by the signal receiving portion 51.
It has been known that an amplifier of a type comprising a semiconductor device to serve as an amplifier device thereof generally encounters change in its gain if the atmospheric temperature is changed. An amplifier of a type comprising a double-gate MOSFET or a bipolar transistor to serve as the amplifier device thereof also encounters the foregoing gain change.
FIG. 11 is a characteristic graph showing the relationship of the gain with respect to gain control voltage in a state where the atmospheric temperature is used as the parameter, the relationship being realized in an AGC amplifier using the double-gate MOSFET shown in FIG. 9.
Referring to FIG. 11, the axis of abscissa stands for gain control voltage (V) to be supplied to the second gate G2 of the double-gate MOSFET, the axis of ordinate stands for gain (db), curve a stands for the characteristics realized when the atmospheric temperature is -20.degree. C., curve b stands for the characteristics realized when the atmospheric temperature is 25.degree. C. and curve c stands for the characteristics when the atmospheric temperature is 80.degree. C.
FIG. 12 is a characteristics graph showing the relationship of the gain with respect to the gain control voltage in a state where the atmospheric temperature is used as the parameter, the relationship being realized in an AGC amplifier using the bipolar transistor difference-amplifying stage shown in FIG. 10.
Referring to FIG. 12, the axis of abscissa stands for gain control voltage (V) to be supplied to the base of the transistor for the electric current source, the axis of ordinate stands for gain (db), curve a stands for the characteristics realized when the atmospheric temperature is 80.degree. C., curve b stands for the characteristics realized when the atmospheric temperature is 25.degree. C. and curve c stands for the characteristics when the atmospheric temperature is -20.degree. C.
As can be understood from characteristics graphs shown in FIGS. 11 and 12, if the environmental temperature of the conventional transmitting/receiving unit is changed considerably, the gains of the first and second AGC amplifiers 56 and 61 are changed considerably as compared with the changes occurring at room temperature in the regions in which the gain control voltage is relatively large in a case where the double-gate MOSFETs 69 and 70 are used in the first and second AGC amplifiers 56 and 61. If the environmental temperature is changed from room temperature in a case where the bipolar transistors 71 to 76 are used, the gains are changed considerably over the entire range of the gain control voltage. In this case, the gains are changed more considerably in a region in which the gain control voltage is relatively small than a region in which the gain control voltage is relatively large.
As described above, if the environmental temperature is changed considerably in a case where the level of the received frequency signal is not changed, the gain of the second AGC amplifier 61 is considerably changed. As a result of the change in the gain, the level of the first AGC voltage V.sub.TAGC is changed as well as the level of the second AGC voltage V.sub.RAGC. Therefore, the gain to be amplified by the first AGC amplifier 56 is changed, thus causing the level of the power for transmitting the frequency signal to be changed.
If the level of the power for transmitting the frequency signal is changed regardless of the change in the level of the received frequency signal, the correspondence between level of the received frequency signal and the level of the power for transmitting the frequency signal is disordered. As a result, the operation of the communication system between movable members encounters a problem.